This relates to security documents, and more particularly to an arrangement for making more difficult the nefarious alteration of security documents printed with a thermal printer. Thermal printers are useful for a wide variety of applications, including printing receipts and credit slips that may later be exchanged for cash. For example, an automated coin counting machine, typically located in a supermarket, issues a thermally printed receipt when a quantity of coins is poured into the machine and counted. The receipt is then taken to a cashier where the bearer will receive the printed amount in paper currency. Clearly, fraudulent alteration of the receipt could result in a significant loss for the supermarket.
A common approach to altering documents printed with a thermal printer is to wash the documents in a solvent that dissolves the heat activated coating with the thermally printed indicia, leaving a clean form document. After the washing process, the documents may be dried and then printed with altered information. Because of the wide use of thermal printers, it is desired to provide thermally printed documents that are not easily altered.